Multimode fibers are successfully used in high-speed data networks together with high-speed sources that are typically using transversally multimode vertical cavity surface emitting lasers, more simply called VCSELs. Multimode fibers operating at 850 nm and 1300 nm are well known.
Multimode fibers are affected by intermodal dispersion, which results from the fact that, in a multimode fiber, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information, but travelling with different propagation velocities. Modal dispersion is expressed in terms of Differential Mode Delay (DMD), which is a measure of the difference in pulse delay (ps/m) between the fastest and slowest modes traversing the fiber.
In order to minimize modal dispersion, the multimode optical fibers used in data communications generally comprise a core, generally doped with Germanium, and showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “α profile,” as follows:
            n      ⁡              (        r        )              =                            n          0                ⁢                              1            -                          2              ⁢                                                Δ                  ⁡                                      (                                          r                      a                                        )                                                  α                                                    ⁢                                  ⁢        for        ⁢                                                  ⁢                                                ⁢        r            ≤      a        ,                where:        n0 is a refractive index on an optical axis of a fiber;        r is a distance from said optical axis;        a is a radius of the core of said fiber;        Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and        α is a non-dimensional parameter, indicative of a shape of the index profile.        
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is thus possible to theoretically obtain a group velocity, which is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength. However, an optimum value of the parameter α is valid for a particular wavelength only.
Graded-index alpha-shape profile and core-cladding interface of the multimode fibers are optimized to operate with GaAs VCSELs that can be directly current-modulated to support 10 Gbps and 25 Gbps systems at 850 nm. Backwards compatibility for use at 1300 nm with LED sources is also guaranteed for most of the 50 μm and 62.5 μm multimode fibers currently in use. The performances of such laser-optimized, high bandwidth 50 μm multimode fibers, also called OM4 fibers, have been standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard.
However, the explosion in demand for bandwidth in enterprise networks is driving an urgent need for higher Ethernet network speeds. To further increase the data bit rate for next generation 400 GbE systems, the use of InGaAs VCSELs operating at 40-56 Gb/s between around 850 nm and 1200 nm combined with wavelength division multiplexing appears as a promising solution, as it will allow achieving both higher speed and higher reliability. In such configuration, OM4 performances are thus required over an extended transmission bandwidth, compared to off-the-shelf OM4 M M Fs, optimized at 850 nm.
Since the optimum value of alpha is wavelength dependent, the transmission bandwidth generally becomes significantly small at other wavelengths than the optimum wavelength. The one skilled in the art knows well that it is possible to use dopants like Phosphorus (P) or Fluorine (F) to modify the refractive index of silica SiO2, and thus allow reducing the wavelength dependence of the optimum alpha.
Patent document U.S. Pat. No. 7,421,172 in the name of Draka Comteq B. V. discloses multimode optical fibers, which graded-index cores are built up by using GeO2 and F as dopants in SiO2. By varying the concentration of dopants over the core radius, the intermode dispersion characteristics of the multimode optical fibers can be adapted in such a manner that the bandwidth is less wavelength-dependent.
Although such a co-doping of graded-index multimode fibers allows achieving a higher bandwidth over a wider wavelength range than for previously known multimode fibers, such a bandwidth is not high enough to meet the demand of high bit rate for next generation systems.
Document U.S. Pat. No. 8,391,661 in the name of Draka Comteq B. V. discloses a multimode fiber with higher modal bandwidth and larger numerical aperture than multimode fibers of the prior art. To this end, such a multimode optical fiber includes a central core that follows a modified power-law equation with an exponent alpha (e.g. an alpha parameter) that depends on radial position within the optical fiber. The alpha parameter has at least two different values along the central core's radius. A first alpha parameter value α1 controls the shape of the graded index core in an inner zone of the central core, and a second alpha parameter value α2 controls the shape of the graded-index core in an outer zone of the central core. The second alpha parameter value is typically less than the first alpha parameter value. The graded-index core profile and its first derivative are typically substantially continuous over the width of the graded-index core. The central core's graded-index profile has a delta Δ value of 1.9 percent or more.
In this U.S. Pat. No. 8,391,661 document, the use of graded index profile with two or more alpha values aims at designing multimode optical fibers having a large numerical aperture NA. However, such high NA multimode optical fibers have bandwidths, which are optimized for a single wavelength (typically 850 nm). They do not allow reaching OM4 performances over an extended transmission bandwidth.
Document U.S. Pat. No. 7,315,677 discloses multimode optical fibers comprising Germania (GeO2) and Fluorine co-doped in the core of the fiber. The dopant concentration profiles are defined by a pair of alpha parameters, α1 and α2. The operating window, or bandwidth window, is enlarged and attenuation, or loss, is low. In some embodiments, two operating windows are available for transmission.
Document U.S. Pat. No. 7,315,677 hence teaches “double alpha profiles” based on co-doping, where each dopant profile is the sum of two alpha profiles with same α1 and α2 used for both compounds. The alpha definition is different from the definition of alpha profile commonly used. Such profiles are difficult to produce from a process point of view. Actually, the concentration shape of Ge and F are difficult to control.
More generally, some solutions consisting of using full-fluorine or low Ge doped concept have been proposed in the literature and in patents. Some of these solutions also teach double alpha profiles, where each dopant profile uses its own alpha. But such solutions require having an outer cladding with a refractive index much lower than the refractive index of SiO2. Thus, for deposition processes like MCVD (for “Modified Chemical Vapor Deposition”) and PCVD (for “Plasma Chemical Vapor Deposition”), which require depositing the different doped layers within a substrate tube, such solutions are quite complex. It is necessary to manage leakage losses and/or to add an outer depressed-cladding. Furthermore, with a Fluorine-doped outer cladding, it becomes difficult to consider “trench-assisted” concepts that require to further F-dope the cladding. It would require F concentration levels that cannot be reached with the existing deposition processes.
It would hence be desirable to design a multimode optical fibre adapted to wideband applications and showing improvements over the prior art.
More precisely, it would be desirable to design a multimode optical fibre, showing OM4 performance increased to multiple wavelengths or to a wavelength-operating window larger than 150 nm.
It would also be desirable to design such a multimode optical fiber, which is easy to manufacture, notably through the use of deposition process like MCVD and PCVD.